Ignition, basic function

With idle speed control active, the timing is adjusted to stabilize the idling speed. The value is sent to box 3.

Fig 1: Illustration For Steps 1-6

Courtesy of SAAB-SCANIA OF AMERICA, INC.

When idle speed control is inactive, the ignition timing is read from a load and engine speed dependent matrix. The value from the matrix is optimized for the lowest fuel consumption (best engine torque). The value is sent to box 3.

One of the ignition timing calculations is selected depending on which function is active. The value is sent to box 6.

In order to heat the catalytic converter as quickly as possible after starting, the ignition will be retarded. This is a compensation matrix that is added to the value from box 3. The matrix is dependent on load and engine speed.

The function is active when the coolant temperature is above -10°C but below +64°C.

The value from box 5 is added to the value from box 3.

The ignition timing is corrected depending on the engine coolant temperature and intake air temperature. The value is sent to box 6.

Fig 2: Illustration For Steps 7-9

Courtesy of SAAB-SCANIA OF AMERICA, INC.

If knocking occurs, a timing retardation will be calculated. The value is sent to box 6.

The compensation angle and knock retardation are totalled to give the current ignition timing. The value is sent to box 7.

Starting ignition timing is selected when the engine has not yet been started. The value is sent to box 9.

Fig 3: Illustration For Steps 10-12

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The starting ignition timing is calculated depending on the intake air temperature and the coolant temperature. The value is sent to box 9 via box 7.

At the calculated crankshaft angle, the microprocessor controls the transistor for the trigger that is next in the firing order.